In many instances it is desirable to stimulate a hydrocarbon producing interval to improve productivity or to remove damage. Several means are available to accomplish the desired stimulation. In carbonate formations such as limestone and dolomite, hydrochloric acid and certain organic acids (e.g. formic, acetic, and oxalic) have been very useful in etching channels, dissolving formation materials in pore spaces, and improving well productivity.
In sandstone formations, HCl/HF acid formulations have been used to remove wellbore damage. This method of damage removal is highly dependent upon the reactivity of HF acid on silicates (especially drilling mud residue and formation clays). HF acid, however, has two characteristics which limit its application. Firstly, HF is very reactive and spends quickly on clays and quartz particles in the sandstone matrix. Secondly, HF can provide reactant by-products such as fluorosilicates and calcium fluoride precipitates which can cause severe formation damage. This damage is not readily removable by chemical means. To limit the side reactions, a pre-flush of HCl is typically pumped into the formation ahead of the HCl/HF fluid. The HCl preflush reacts with any acid soluble calcium or magnesium minerals (calcite and dolomite, in particular) in the sandstone prior to the arrival of the HCl/HF acid. The HCl component of HCl/HF acid also helps keep the pH of the spent HF acid low which also helps prevent precipitation of HF reactant by-products.
Therefore, what is needed is a method for acidizing a formation where the acid utilized maintains its strength while avoiding the buildup of harmful precipitates and thereby allows for greater penetration into the formation than heretofore possible.